Neutron scattering experimentation has been conducted both in the United States and throughout the world since the early 1950's. The fundamental breakthroughs in neutron research are the result of the expenditure of hundreds of millions of dollars in the development and construction of research and analytical facilities. Neutron research has found its greatest utility in applications such as elemental analysis, determination of atomic arrangement, the magnitude and direction of atomic magnetic moments, and the examination of macroscopic bodies for structural flaws. In fact, neutron experimentation/analysis is often the only source of essential analytical information, which is otherwise unattainable by other spectroscopic or diffraction techniques. Neutron research is also critical to the development of advanced synthetic materials for use in a host of "next generation" products and applications. The basic principles of neutron reflection are reported on in a paper by G. P. Felcher entitled "Principles of Neutron Reflection" SPIE Vol 983, Thin-Film Neutron Optical Devices (1988).
Several laboratories (including many of the United States national labs) have proposed improving the quality and quantity of neutron experimentation by channelling neutrons to remotely located experimental stations via neutron reflecting guidetubes. The advantages of this approach have been demonstrated by research facilities in Europe and Japan. Heretofore, the only method for collecting and transporting neutrons (particularly those neutrons having the most utility, and characterized by wavelengths of greater than about 0.4 nm, also known as "cold neutrons") was to employ guidetubes having 100 nm thick nickel plated glass plates disposed therein. Nickel has, until recently, been a preferred reflective element for use in neutron guidetubes due to the fact that nickel has the maximum reflection angle of any single element. In fact, nickel's maximum reflection angle is approximately equal to the critical reflection angle for a neutron wavelength of 0.4 nm (i.e. approximately 0.4 degrees theta). This critical angle is important since it defines the angular acceptance of the guidetube, and since the neutron flux from a guidetube is typically measured in "counts per minute" (as opposed to x-ray fluxes in excess of 1000 counts per second) enhanced guidetube acceptance is highly desirable.
Recently, much interest has been shown to the area of multilayered neutron reflecting supermirrors for improving neutron acceptance and throughput beyond that of pure Ni films. These supermirrors typically take the form of layered films of titanium and nickel having a distribution of bilayer thicknesses designed to yield overlapping Bragg diffraction peaks to occur from the region just above the cutoff angle characteristic of Ni to some increased angle of acceptance. It is also important that reflectivity of the supermirror remains high as the reflection angle is extended, otherwise the cumulative reduction from each reflection along the guidetube would result in an unacceptable loss of neutron flux. Heretofore, titanium and nickel have been the preferred choices for non-polarizing supermirrors owing to their high characteristic effective neutron scattering.
Experimental Ni-Ti supermirror guidetubes have found limited use in Japan and Europe as was demonstrated by Ebisawa, et al in a publication entitled "Nickel Mirror and Supermirror Neutron Guides at the Kyoto University Research Reactor" SPIE Proc., v. 983, pp 54-58, (1988); and Schoupf, "Recent Advances with Supermirror Polarizers," AIP Proc., No. 89, pp 182-189 (1982). Other progress has been reported in a publication by Rossback, et al entitled "The Use of Focusing Supermirror Neutron Guides to Enhance Cold Neutron Fluence Rates" Nuclear Instruments and Methods in Physics Research B 35 (1988) 181-190. Rossbach, et al report improved neutron reflecting characteristics in carefully deposited Ti-Ni supermirror structures on certain glass substrates having a measured roughness of 18.5 angstroms. These supermirror structures have been used in neutron reflecting applications requiring reflectivity lower than standard guidetubes: hence reflectivity is approximately 65%. Rossback, et al indicates that layer imperfections resulting from crystal growth are the primary reason for less than optimum reflectivity.
While Ti-Ni supermirrors have to date proven effective for use in neutron reflecting applications, routineers skilled in the supermirror art have reported observing distortions in the layer structure. The result is a loss (often serious) in the reflectivity of the supermirror, and hence a significant increase in the effective cost of neutron experimentation. It is believed that the layer distortions are the result of, inter alia, crystal growth in the layers, materials interactions, film stresses of the layered structures and/or interdiffusion of Ti and Ni at the layer interfaces which prevent the attainment of the high degrees of layer flatness required in order to achieve the desired high reflectivity. In fact, the instant inventors have found that a lack of layer flatness is the primary cause for reduction in reflectivity.
Attempts to enhance supermirror performance so as to achieve neutron reflection of greater than three times the critical angle of nickel alone have been reported in a number of publications. Prior to the invention of the novel supermirror structures described hereinbelow, no supermirrors have achieved the performance levels needed for practical neutron guidetube applications. Numerous reasons for the poor performance of supermirrors have been advanced, as in, for example, a publication by Keem, et al entitled "Neutron, X-Ray Scattering and TEM Studies of Ni-Ti Multilayers" published in SPIE Vol. 983 Thin-Film Neutron Optical Devices (1988) which identifies cusp formation in the Ni-Ti bilayers as the principle factor preventing supermirror performance at acceptable levels.
Progress has, however, been reported in the development and fabrication of polarizing (i.e. supermirrors which more effectively reflect one polarization of neutron spin) supermirrors. Mook and Hayter report in "Transmission Optical Device to Produce Intense Polarized Neutron Beams" Appl. Phys. Lett. 53 (8), 22 Aug. 1988, p. 648 a highly effective polarizing neutron mirror making use of a crystalline silicon layer for polarizing reflected neutrons. However, improvements in the field of polarizing neutron mirrors are only marginally applicable in the fabrication of non-polarizing supermirrors.
Accordingly, there exists a need for an improved neutron reflecting supermirror structure characterized by neutron reflectivities in excess of 97% and a critical angle of at least two times the critical angle of standard neutron reflectors such as nickel or Ti-Ni alloys. These improved supermirror structures will, of course, have to overcome the reflectivity problems resulting from a number of technical problems, including but not limited to, lack of layer flatness caused by, for example, crystal growth.